This invention relates to fasteners and self-tapping fasteners that form internal threads using a swaging or roll forming process. More particularly the invention relates to a fastening element and method capable of forming a fastener assembly by engagement with a self-tapping fastener that reduces the required end load to start the tapping process and assists in the proper alignment of the self-tapping fastener.
Self-tapping fasteners such as self-tapping screws or bolts fall into two broad classes. The first are those which are provided with cutting edges at the work entering end. The second and most common type are those which are so designed to form uniform load carrying internal threads into untapped fasteners or pilot holes with a swaging operation. Fasteners of the first type have numerous disadvantages and one of the most significant being that they all form chips which are cut from the body to which they are driven. As a result, self-tapping fasteners that form threads by deforming a thread pattern within a pilot hole have become the most popular design. Such fasteners are available from a variety of sources and are marketed under the trademark TAPTITE(copyright) in connection with a trilobular or three-lobe thread forming blank design.
FIGS. 1-2 illustrate a conventional three-lobed fastener according to the prior art. All threads have a characteristic pitch and diameter because of the lobulation of the threads, the radial offset from the axis will vary about the circumference. In general, standard thread diameters and pitches are provided to lobular fasteners, but the lobes tend to have a slightly larger diameter than a standard thread diameter. This enable the lobes to positively form corresponding internal threads as the fastener is driven into an appropriately sized pilot hole into the shape of conforming internal threads.
As the fastener is rotated the lobes engage the inner wall of the pilot hole (not shown) and begin to displace material within the pilot hole. In a typical self-tapping fastener, the threaded fastener is provided with a stabilizing zone having stabilizing threads at the end of a fastener shaft and a thread forming zone with corresponding thread forming threads along the shaft of the fastener adjacent the stabilizing zone. The stabilizing zone as illustrated in FIGS. 1-2 often has a reduced diameter enabling it to fit within an initial untapped hole in a relatively perpendicular fashion. The thread forming zone often has a sloped or tapered shape with a diameter that increases linearly between the stabilizing zone and the full diameter main body of the fastener.
Prior known constructions have often provided the thread stabilizing zone and the thread forming zone with a higher out of round than the full diameter main body. In one example, the out of round of the thread forming zone gradually tapers back from the highest out of round adjacent to the stabilizing zone toward the lower out of round that defines the full diameter main body. In another often preferred example, the thread forming zone can define an approximately constant profile high out of round along its entire axial length that transitions step wise at the main body into the characteristic lower out of round. In connection with either example, there is a difference between the high out of round at the stabilizing section and at the main body cross section.
As a self-tapping fastener is driven into an untapped pilot hole the thread forming threads encounter the sidewalls of the hole initially. These threads often exhibit an increasing outer diameter and higher out of round. As such, the lobes are able to gradually apply increasing thread forming pressure to the pilot hole until each formed internal thread is contacted by the first full diameter thread. This first full diameter thread often has the out of round profile of the rest of the main body. It provides final formation of each thread in the pilot hole to the desired shape.
Self-tapping threaded fasteners are frequently preferred in applications where it is possible to use a metal screw which is harder than the material of a mating element such as a blank or nut through which a threadless bore for the screw has been made. In general, properly forming internal threads in a bore requires several swaging blows from the underlying lobes of the fastener. This process, in essence, forms a shape in the ductile metal of the untapped pilot hole or fastener corresponding to the threads of the self-tapping fastener. A sufficient number of forming threads is necessary to complete the process. Depending upon the nature and hardness of the metal into which a self-tapping fastener is driven, a relatively high driving torque is usually required, particularly in metal having an appreciable thickness. This often results in a stripping torque to driving torque ratio that is relatively low. The requirement of high driving torque not only creates problems with respect to drivability but a low driving torque to stripping torque ratio can restrict the usage of automated power drivers in assembly lines.
It is well known that the driving torque of individual fasteners can vary considerably due to the presence of any lubricant, slight variations in the material hardness into which the fastener is driven, in the hole size, in the fastener diameter, as well as dullness of cutting edges or from misformed or damaged threads (especially the lead threads) from handling or processing such as plating. Similarly, failure torque, including stripping torque of the mating threads as well as the failure torque of the fasteners themselves can vary somewhat considerably from one fastener to the next. The clutch or related mechanisms of the power drivers cannot be relied upon to disengage at precisely the same torque value each time. If the driver is set just above the normal driving torque, and any of these variations causes an increase driving torque, conventional tapping fasteners will not be driven in fully and loose assemblies could result. If the driver clutch is so adjusted to give a greater driving torque so as to overcome any such difficulty, a conventional tapping fastener can then be overdriven, resulting in stripped threads or broken fasteners, either of which will result in costly delays of the assembly line while repair or replacement is made.
It is also known, that in many cases the efficiency and thus the usefulness of self-tapping operation can be problematic, particularly because at the beginning of each operation considerable pressure or end load must be applied by means of a conventionally used power driven tool to cause the self-tapping screw to properly start winding itself into the material adjacent the cylindrical surface defining the threadless bore. Such forces can make proper alignment difficult. Difficulties may be encountered when the bore is originally, or thereafter becomes oriented at an angle relative to a driven self-tapping fastener such that the fastener is not in perfect alignment with the axis of the bore. As a result, the fastener may become permanently askew and not seat properly. This can be where the lead thread of the fastener is initially slightly misformed or thereafter becomes distorted.
Such problems have been acute where for example, the bore axis extends horizontally and the self-tapping fastener is driven from a position relatively higher than or relatively lower than the axis. In many such instances, the threads of the self-tapping fastener which are designed to form threads within the bore upon proper engagement then are mangled or otherwise distorted. If the resulting assembly is formed at all, it may have significantly impaired holding characteristics since the underside of the fastener itself may be damaged and thus weakened. Additionally, the entire fastening assembly may be weakened and put in jeopardy. Moreover, the cocked or askew fastener head may have roughened the surface of the structural element containing the bores such that it would not hold paint, or such that the thickness of such element may be reduced and consequently the entire assembly may become weak. The askew screw head appearance also is undesirable. Frequently, in such situations a new fastener must be driven into the bore, new bore formed, or the part must be scrapped entirely.
In order to try and overcome these drawbacks and to make the process go more quickly, a high out of round, which concentrates the force of the blows generated by the underlying lobes of the fastener has often been utilized. Use of a high out of round within the main full diameter threads, reduces the amount of torque that must be applied to form threads. However, this lower torque comes at a price, since it results in less diametrical material remaining in contact with the internal thread once it is formed. Hence, such fasteners will not hold as much load as a more round fastener. This increases the chances of failure occurring in such a fastener system. Such failure in general results from axial pull out, or when thicker nut members are used, fracture. Also, since area varies by the square of the radius, the use of a higher out of round cross-section results in a significantly reduced cross-sectional area, which lowers the screws failure limit. Hence, self-tapping screws typically use an out of round dimension that is a compromise between the optimum value for thread forming efficiency and the optimum value for resistance to failure.
Another drawback of self-tapping fasteners is that in order to engage a pilot hole and begin forming threads, they necessarily are initially pulled somewhat out of proper alignment. If the thread forming fastener does not start in a straight line like a normal threaded bolt and nut combination for example, then the threads can be improperly formed and can pose further problems if the fastener is ever removed and then reinserted, since cross threading or additional thread cuts will then likely result. It is the inherent nature of a thread forming fastener to start out of alignment and subsequently straighten up. In order to accomplish this, the undesirable application of significant additional torque to drive the fastener is often required. In some castings with unthreaded bores this has lead to cracking of the casting itself.
To date, great effort has been placed into modifying the geometry construction of self-tapping fasteners such as screws or bolts in order to try to overcome these above stated problems, but they have still left significant issues or compromises. Since most all self-tapping fasteners are designed to create uniform load carrying internal threads into untapped nut members or other similar bores upon installation, the structure and the geometry of the untapped bore has not been given equal attention as a potential solution to these problems. Most modifications to unthreaded nuts or mating type fasteners have been directed to nuts that have a particular structure that assists in aligning the screw or bolt that is to be mated with the self-tapping fastener. Known solutions directed to threadless nut type fasteners have generally involved extensive and complicated geometries that project inwardly from the untapped sidewalls and have not decreased the required driving torque and are cumbersome and expensive to form.
Several solutions involving a fastening element designed to form a fastener assembly by engagement with a self-tapping screw have been proposed. One such construction provides a threadless bore having a varied diameter and an inwardly protruding rib that has at least one interruption therein. This rib, however, extends substantially around the 360xc2x0 circumference. Such a construction involves considerable complication and expense in forming the rib and also requires the self-tapping fastener to remove or form an internal thread through the rib itself.
Another known self threading fastener device for use with a threaded member utilizes a generally helical rib formed from the material of the side wall protruding inwardly from the side wall. The rib is helically inclined so its angle of inclination corresponds generally to the angle of inclination of the threads on the threaded member. The rib must span the entire circumference of a section of the unthreaded bore. This design is quite complicated and therefore, expensive to form and again necessitates the thread forming fastener to engage and cut threads through the inwardly projecting helical member. This has often lead to increasing the required end load or force required to start the tapping process and cracking the fastening device.
It is apparent from the drawbacks of the prior known constructions set forth above that there exists a need for an improved threadless fastening element for use with a self-tapping fastener that overcomes these drawbacks and provides additional benefits and advantages.
In accordance with a first aspect of the invention, there is provided fastening element and method for forming a fastener assembly by engagement with a self tapping fastener comprising a solid body having a threadless internal substantially cylindrical surface defining a bore having an axis and extending through all or a portion of a solid body, and at least one indentation, preferably of a substantially elliptical configuration extending inwardly from a portion of the top of the bore or a lead in to the bore. The indentation extends around the circumference of the cylindrical inner surface from about 1xc2x0 to greater than 360xc2x0 and in some preferred embodiments from about 15xc2x0 to about 360xc2x0 in accordance with certain aspects of the invention. The helix angle or pitch of the indentation of the threadless bore can be specially dimensioned for engagement by a standard size self tapping fastener. More specifically, the indentation of the threadless bore may take the form of a narrow scribe like line having a flat, notched, rounded or angled base or a wider notch that is approximately equal to or greater than the distance between threads of the self tapping fastener.
A further aspect of certain embodiments is to provide a plurality of indentations which in total extend less than the entire 360xc2x0 circumference of the threadless bore. Regardless of the type or number of indentations used, all the indentations can extend only a very slight depth into the internal surface of the thread bore, and in most all cases, significantly less than the depth of the thread to be formed by the self tapping fastener.
The indentation provided in the internal surface of the threadless bore can also initially act as a guide and alignment device for the self tapping fastener. Upon assembly, the end load or force required to start the tapping process is significantly reduced as the self tapping fastener passes the indentation and engages the portion of the threadless bore without the indentation forming threads in the bore. As a result the differential between the driving torque and the failure torque of the self tapping fastener is significantly altered, thereby resulting in fewer failures and significantly reducing the risk thereof while assisting in monitoring proper alignment.
It is therefore a primary object of the present invention to provide a new and improved fastening element for forming a fastener assembly by engagement with a self tapping fastener as set forth above that assists in aligning the fastener and/or decreases the end load required to start forming threads in the element.